when white people clap/snap/count
haha that sounds kinda racist but it's not really. it just seems to be the way things go.

music joke:
how do you find a white guy in the crowd at a jazz gig?
he's the guy snapping on 1 and 3

In unformal terms the phrase "down beat" refers to the very first beat of the first measure of a song or a musical passage. In an orchestra if a particular instrument or section has an extended period of rest (where they arent playing) the first beat of the first measure of the next phrase of music where they begin playing again is referred to as the down beat for that passage.

If you ever observe an orchestra or a jazz ensamble perform, the director will always keep the band in sync by swinging his hand or baton in the downward position indicating that this is where the first beat of the next measure begins.

A bar of music is typically made up of four beats. Each group of two beats is further divided into one down-beat and one up-beat.

So, in the case of a 4/4 time signature:

1(down) 2(up) 3(down) 4(up)

These two groups of down-beats and up-beats are known as "rhythm units". Imagine tapping your foot in time to a piece of music. The down-beats are the beats where your foot hits the floor.

In 3/4 time signature (three beats to the bar) there is one down-beat followed by two up-beats.

The first beat of the bar (measure). It is the strongest beat of the bar, and thus given the most emphasis by the downward movement in a conducting pattern.

A downbeat is the very first beat of each measure, and in most cases it has more emphasis placed on it than the others.